1. Field of the Invention
This invention relates to a method of producing an extra low iron loss grain oriented silicon steel sheet, and more particularly it is to conduct refinement of magnetic domains and hence advantageous improvement of iron loss properties by subjecting a coating layer formed after finish annealing or a mirror finished steel sheet surface after finish annealing to CVD, ion plating or iron implantation process with nitride, carbide, oxide or the like, forming an insulation coating on the resulting tension layer and then subjecting the coating to electron beam (EB) irradiation in a direction crossing the rolling direction.
2. Related Art Statement
Lately, remarkable developments and efforts for satisfying the improvement of electrical and magnetic properties in grain oriented silicon steels, particularly ultimate demand on reduction of iron loss are gradually producing good results.
As is well-known, the grain oriented silicon steel sheet, wherein secondary recrystallized grains are highly aligned in {110}&lt;001&gt; orientation, namely Goss orientation, is mainly used as a core for transformer and other electrical machinery and equipment. In this case, it is required that the magnetic flux density (represented by B.sub.10 value) is high, and,the iron loss (represented by W.sub.17/50 value) is low.
Since these grain oriented silicon steel sheets are usually manufactured through many complicated steps, a great of inventions and improvements are applied to the above steps, whereby low iron loss grain oriented silicon steel sheets having B.sub.10 of not less than 1.90 T and W.sub.17/50 of not more than 1.05 W/kg when the product thickness is 0.30 mm or B.sub.10 of not less than 1.89 T and W.sub.17/50 of not more than 0.90 W/kg when the product thickness is 0.23 mm are manufactured up to the present.
Lately, supreme demands on the reduction of power loss become considerable in view of energy-saving. Particularly, a system of "Loss Evaluation" wherein the reduction percentage of iron loss is converted into a money to load on the cost of the transformer in the manufacture of low loss transformer is widely spread in Europe and America.
Under the above circumstances, there has recently been proposed a method wherein local microstrain is introduced into the surface of the grain oriented silicon steel sheet by irradiating a laser beam onto the steel sheet surface in a direction substantially perpendicular to the rolling direction after the finish annealing to thereby conduct refinement of magnetic domains and hence reduce the iron loss (Japanese Patent Application Publication Nos. 57-2,252, 57-53,419, 58-26,405 and 58-26,406).
Such a magnetic domain refinement is effective for the grain oriented silicon steel sheet not subjected to the strain relief annealing, in the manufacture of stacked lamination core type transformers. However, in case of wound-core type transformers, the strain relief annealing is performed after the magnetic domain refinement, so that the local microstrain produced by laser irradiation on purpose is released by the annealing treatment to make the width of magnetic domains wide and consequently the laser irradiating effect is lost.
On the other hand, Japanese Patent Application Publication No. 52-24,499 discloses a method of producing an extra-low iron loss grain oriented silicon steel sheet wherein the surface of the grain oriented silicon steel sheet is subjected to a mirror finishing after the finish annealing or a metal plating is applied to the mirror finished surface or further an insulation coating is baked thereon.
However, the mirror finishing for improving the iron loss does not sufficiently contribute to the reduction of iron loss in comparison with remarkable cost-up of the manufacturing step. Particularly, there is a problem on the adhesion property to the insulation coating indispensably applied and baked after the mirror finishing. Therefore, such a mirror finishing is not yet adopted in the present manufacturing step.
Further, there is proposed a method, wherein the steel sheet surface is subjected to the mirror finishing and then a thin coat of oxide ceramics is deposited thereon, in Japanese Patent Application Publication No. 56-4,150. In this method, however, the ceramic coat is peeled off from the steel sheet surface when subjecting to a high temperature annealing above 600.degree. C., so that it can not be adopted in the actual manufacturing step.
Moreover, Japanese Patent laid open No. 59-229,419 proposes a method wherein a heat energy is locally applied to the surface of the silicon steel sheet to form a heat strain zone. However, the effect based on the preferential formation of such a local heat strain zone is lost by high temperature annealing above 600.degree. C. In addition, there are proposed a method of introducing artificial grain boundary into the silicon steel sheet having a secondary grain size of not less than 3 mm in Japanese Patent laid open No. 58-144,424 and a method of irradiating plasma flame to the grain oriented silicon steel sheet after the finish annealing in Japanese Patent laid open No. 62-96,617. In the latter methods, however, the effect is lost in case of the material for Wound-core type transformer subjected to the strain relief annealing.